Ocular tension is adjusted by the amount of aqueous humor filling the aqueous chambers. When the production of aqueous humor is increased or excretion of aqueous humor is inhibited, the ocular tension is increased. For example, glaucoma is an ophthalmic disease in which excavatio disci nervi optici and/or visual field defect is caused by the continues high ocular tension, which leads to blindness unless the high ocular tension is well cured. Further, in a type of glaucoma, various symptoms such as visual field defect occur under the ocular tension which is considered to be physiologically normal. It is thought that this is caused by the fact that pressure sensitivity of the visual nerves of the individual is high, and the disease is called low tension glaucoma. Inversely, some individuals show abnormally high ocular tensions while no clear lesions are observed in the visual nerves. This state is called hypertonia oculi or ocular hypertension. Agents for decreasing ocular tension are useful for therapy of various high ocular tension states such as glaucoma, ocular hypertension and high ocular tension which occurs after surgery.
In recent years, it was reported that prostaglandin-related compounds have activities to decrease ocular tension (13,14-dihydro-15-keto-prostaglandins: Japanese Laid-open Patent Application (Kokai) No. 2-108; 13,14-dihydro-15(R) 17-phenyl-18,19,20-trinor prostaglandin F.sub.2a ester: Japanese Laid-open Patent Application (Kokai) No. 6-500804), so that prostaglandins draw attention as drugs for decreasing ocular tension. It has been reported that prostaglandin I.sub.2 (PGI.sub.2, prostacyclin, Nature, Vol.268, p.688, 1976) and a derivative thereof, Iloprost (Philip F. J. Hoyng et al., 1989) have activities to decrease ocular tension in experimental animals.
However, it is known that prostaglandin I.sub.2 and Iloprost cause transient increase in ocular tension and/or hyperemia. The object of the present invention is to provide an agent for decreasing ocular tension free from such side effects and has a high effectiveness.
PGI.sub.2 has a drawback in that it is unstable in vivo so that its physiological activity does not last. PGI.sub.2 derivatives with which this drawback is largely improved, which have a structure wherein the exoenol ether moiety is converted to inter-m-phenylene are described in Japanese Patent No. 1933167. It is known that these PGI.sub.2 derivatives have a platelet aggregation-inhibition activity, vasodilating activity, gastric-acid secretion-inhibition activity, bronchodilating activity, uterine-contracting activity and the like.